This invention relates generally to the field of modular telephone protector devices employed in telephone offices for protecting individual telephone circuits, as exemplified by U.S. Pat. No. 3,947,730 granted Mar. 19, 1976 and assigned to the same assignee as the instant application.
Devices of this type are normally provided with a heat-sensitive spring-loaded structure which, upon the occurrence of a continued surge of excessive current in the line, serve to ground the line to thereby discharge the current. As each telephone circuit includes a so-called "ring" line operating on one voltage, and a "tip" line for conversation, it is customary to protect both lines by separate heat-sensitive devices incorporated in the module, although it is also known to provide means whereby upon the actuation of one heat-sensitive device, a part of the overload current is diverted to assist in the actuation of the other. In some devices, means is provided whereby upon the grounding of an affected tip or ring circuit, the other circuit is opened.
Most protective modules employ arcing devices, usually in the form of a pair of carbon blocks defining a gap. Arcing devices are useful for momentary surges of excessive voltage, as, for example, when the line is struck by a flash of lightning. However, in the case of a continuous surge of excessive current, as might happen when a power cable falls upon the telephone line, the excessive flow of current creates heat which will destroy the telephone line unless such current is promptly grounded. It is known in the art to employ heat-sensitive devices which will cause such grounding, bypassing the arcing device. It is also known in the art to use gaseous discharge devices in lieu of the spaced carbon blocks. This later device has been refined to provide the equivalent of a pair of gas tubes with a common ground contact. The tube is mounted transversely in the module casing element.